Such an adjustment means is known from U.S. Pat. No. 4,880,271. In that case the support element consists of two parallel longitudinal strips firmly interconnected by means of a plurality of transverse strips. Centrally to the longitudinal strips a further sheet metal strip is fixed at its upper end to the uppermost extreme transverse strip of the support element and is also connected to the cams of an excenter device provided at the lower end of the support element, but not to the subsequent transverse strips. The excenter and thus also the cam is adjustable by way of a worm gear by an operating means comprising a handle, in such a manner that the central sheet metal strip, depending on the direction of rotation, brings about an arching or a flattening (detensioning), as the case may be, of the support element such that the adjustment of the desired degree of arching can be brought about. To the rear of the support element at the upper and lower ends thereof a spring is linked up which draws the two ends of the slightly pre-arched support element towards one another, whereby the adjustment in the sense of an increased arching is supported and thereby facilitated. In this context the spring acts permanently onto the support elementxe2x80x94even if no adjustment takes placexe2x80x94and is accordingly independent of the operating means.
In addition, the entire mechanism of the arching means is extremely complicated. Furthermore, which ever degree of arching is set up this is stiff and unyielding, whereby comfort is adversely affected.
It is an object of the invention to so improve an apparatus of the type referred to in the introduction that inspite of simple construction an easy adjustment of the arching is possible and/or that the tensioning force of any particular set up degree of arching can be adjusted more flexibly, the arched region being moreover able to yield.
As shown by the preferred embodiment adjustment force can be varied by the spring and a certain resiliency of the support element is ensured, i.e., it is not rigid and is able to yield to relatively strong forces whereby the comfort level is increased.
The adjustment forces for arching the support element can be compensated for, virtually completely even, by the optional selection of the spring force, so that the adjustment of the degree of arching can proceed virtually without force application.
Further advantageous embodiments of the invention will be apparent from the further subsidiary claims.